1. Field of the Invention
The present invention relates to a zoom lens, and more particularly, to a low cost zoom lens.
2. Description of Related Art
With the rapid development of video technologies, imaging devices such as digital video cameras (DVC) and digital cameras (DC) have found widespread applications. The core element of these imaging devices is a zoom lens. By varying the optical focus of the zoom lens, a clean image can be focused on a charge-coupled device (CCD). The optical quality of the zoom lens is closely related to the imaging quality, manufacturers are now trying to improve the quality of the zoom lens while lowering the production cost in order to provide a competitive edge of their imaging device in the market.
FIG. 1A is a diagram showing the structure of a conventional zoom lens. The conventional zoom lens 100a in FIG. 1A has been disclosed in the U.S. Patent application 2004/0189834 A1. The zoom lens 110a includes four lens groups 110a, 120a, 130a and 140a. The lens group 110a comprises a single lens 112a; the lens group 120a comprises two lenses 122a; the lens group 130a comprises a single lens 132a; and, the lens group 140a comprises two lenses 142a. 
The conventional zoom lens 100a uses the relative motion between the lens group 110a, the lens group 120a and the lens group 130a to increase the magnification or decrease the magnification. Thus, the zoom lens 110a must have a linkage designed to move the lens groups 110a, 120a and 130a simultaneously. This mechanical linkage not only increases the complexity of the zoom lens, but also increases the production cost and the bulk of the zoom lens.
Furthermore, because a total of 6 lenses are required in the zoom lens 100a, sufficient space must be allowed for accommodating the relative movement between the lens groups 110a, 120a and 130a. As a result, the overall length of the zoom lens 100a is often too large. Aside from the difficulty of miniaturizing the zoom lens, it is also difficult to lower the production cost.
FIG. 1B is a diagram showing the structure of another conventional zoom lens. The conventional zoom lens 100b in FIG. 1B has been disclosed in the Taiwan Patent No. 299039. The zoom lens 100b includes four lens groups 110b, 120b, 130b and 140b whose refractive powers are negative, positive, negative and negative respectively. The lens group 110b comprises three lenses 112b; the lens group 120b comprises three lenses 122b; the lens group 130b comprises three lenses 132b; and the lens group 140b comprises a single lens 142b. 
The zoom lens 100b requires a series of 10 lenses, overall length of the zoom lens is often too large. Thus, not only is miniaturizing the zoom lens difficult, but lowering the production cost of the zoom lens is also difficult as well.